The present invention relates to a method and apparatus for measuring the chromatic dispersion coefficient, particularly that of an optical fiber.
Advances in performance of optical fibers have led to the realization of actual long-distance fiber optical communication systems. A fiber optical communication system, particularly one using a single mode optical fiber for the optical transmission line, has the advantage of extremely small expansion of the pulse width during transmission of the pulses from the transmitter side to the receiver side. Accordingly, such a system is suitable for the transmission of high frequency data, i.e., large amounts of data. With a transmitter using a semiconductor laser as a light source, however, the emission spectrum distribution may be considerably wide and jitter of the pulses may occur. In such a case, with a transmission distance over 50 km, reliable data transmission at a high transmission speed of over 400 Mb/s cannot be ensured since both the expansion of the pulse width and variations in the optical pulses, caused by so-called "chromatic dispersion", reach non-negligible levels. That is, inter-symbol interference and mode dispersion noise increase significantly.
"Chromatic dispersion" specifically refers to the dispersion of the optical pulse distribution at the receiver side and is caused by the inevitable differences in arrival times of various optical pulses at the receiver side in accordance with their respective wavelengths .lambda..sub.1, .lambda..sub.2, .lambda..sub.3 and so on. For example, when a semiconductor laser is used as the light source, three or four wavelengths of light are generated therefrom.
In view of the above, it is very important in the design and construction of a fiber optical communication system to determine the degree of chromatic dispersion of the optical fiber, i.e., the chromatic dispersion coefficient. The chromatic dispersion coefficient determines the quality of the optical fiber transmission line and is used as a standard to determine the data transmission capability.
Various types of systems for measuring the chromatic dispersion coefficient are known, for example, a measuring system utilizing the Raman effect and a measuring system operated under a frequency sweep method. Each of the prior art measuring systems have their own shortcomings, however.